The Pima Indian population of Arizona has one of the highest reported prevalence rates of obesity in the world. To determine whether a "thrifty gene" is the genetic defect predisposing the Pima Indians to obesity, we have investigated different components of the total daily energy expenditure in both Pima Indians and Caucasians. Basal metabolic rates, glucose-induced thermogenesis, and response to exercise have been measured by an open-circuit hood system indirect calorimeter, whereas our new respiratory chamber has been used to measure the major determinants of 24-hour energy expenditure. Furthermore, we are planning to use this chamber to investigate the short-term response to over- and underfeeding in terms of energy expenditure and substrate oxidation in lean and obese Pima Indians. Measurements of different components of energy expenditure have been performed in siblings in order to assess the importance of genetic factors in the overall energy balance.